Water supplies stressed

Groundwater resources have shrunk by 1,000 cubic kilometers since 1900 as the amount of water extracted exceeds the rate at which aquifers are recharged, according to “Groundwater depletion in the United States.”

By:
John Kemp, Reuters

U.S. farmers are withdrawing unsustainable volumes of groundwater to irrigate their crops, resulting in an accelerating decline in aquifers across the central and western United States, according to a new report by the U.S. Geological Survey.

Groundwater resources have shrunk by 1,000 cubic kilometers since 1900 as the amount of water extracted exceeds the rate at which aquifers are recharged, according to “Groundwater depletion in the United States.” This is about double the total amount of water contained in Lake Erie.

And depletion rates are accelerating. The 40 aquifers in the USGS survey declined almost 200 cubic kilometers from 2000 to 2008 alone — a record rate of 24 cubic kilometers per year, which is more than double the 11 cubic kilometers per year lost in the 1990s and the 12 lost each year in the 1980s.

Falling Ogallala aquifer

The giant High Plains or Ogallala aquifer, which underlies about 450,000 square kilometers of the central U.S. including parts of Texas, New Mexico, Oklahoma, Kansas, Colorado, Nebraska, Wyoming and South Dakota, has lost more than 340 cubic kilometers of water since the beginning of the 20th century.

Nearly 25 percent of the total — 82 cubic — was lost in 2000 to 2008 alone, the most recent eight-year period covered by the survey.

In the worst-affected parts of northern Texas, so much water has been pumped from the High Plains aquifer that the water table has fallen by more than 150 feet.

Environmentalists blame hydraulic fracturing for adding to the stress in drought-hit areas. Fracking each well can require millions of gallons of water. But groundwater depletion started to accelerate in the 1940s and 1950s, long before fracking became widespread in the 2000s.

The bigger problem is water-intensive agriculture. In 2005, about 80 billion gallons of groundwater were withdrawn every day across the U.S., of which 53.5 billion were used for irrigation — dwarfing the 14.6 billion gallons used for domestic supply or the 3 billion gallons used by industry.

In Texas, irrigation accounted for more than 71 percent of all groundwater extraction, rising to 95 percent in Nebraska. (“Estimated water use in the United States,” 2009).

The massive rise in water use for irrigation can be attributed mainly to the introduction of center-pivot, sprinkler systems in the 1950s.

In North Dakota, which draws most of its water supply from its own giant aquifers, the use of groundwater for irrigation has risen steadily since center-pivot systems started to become common.

According to one research report, the number of irrigation wells in the state rose from fewer than six in 1960 to almost 1,500 by 1980, which were pumping up to 65 million gallons per day at the height of the irrigation season to water 100,000 acres of land (“Guide to North Dakota’s Ground Water Resources,” 1983).

Over-pumping is not a new problem. From 1900 to 1930, so many artesian wells were drilled in southeast North Dakota and eastern South Dakota and allowed to flow freely that the water pressure plunged and states had to pass laws obliging all wells to be fitted with pressure control valves.

But the withdrawals are now on an unimaginable scale.

The problem is compounded because the recharge rates for the High Plains/Ogallala aquifer are low. Figures for 30-year average annual precipitation range from 36 centimeters in the west to about 81 centimeters in the eastern part of the aquifer.

“Evaporation rates are high relative to precipitation, so there is little water available to recharge the aquifer,” according to USGS. “Potential recharge in nonirrigated areas has been estimated to range from less than 6 millimeters along the western boundary of the aquifer to as much as 127 millimeters in the northeastern part of the High Plains aquifer.”

Farmers and frackers

“Continuation of depletion at observed rates makes the water supply unsustainable in the long term ... observed rates of depletion must eventually decrease as economic and physical constraints lead to reduced levels of extraction,” USGS notes.

As Herbert Stein, Richard Nixon’s chief economic adviser, noted: if something is unsustainable, it will stop.

The rate of depletion is leveling off in some areas or becoming “self-limiting” as constraints set in, according to USGS. Nationwide, however, depletion rates continue to accelerate unsustainably.

Depletion on this scale represents a severe threat to the environment, as well as to supplies of water for drinking and irrigation on which communities depend. Once depleted, the lost storage cannot easily or quickly be recovered, USGS warns.

Fracking opponents like to cite the large volumes of water needed to stimulate oil and gas production by pressure pumping to hydraulically fracture rock formations deep underground. Frackers’ water use is highly visible because the water is normally brought in by the truckload.

In reality, the main pressure comes from farmers. If fracking adds to the pressure on local water supplies at the margin, it is only because intensive agriculture is already rapidly sinking the water table across large parts of the U.S.

“Groundwater depletion in the United States 1900-2008” is available from the USGS website: pubs.usgs.gov.